I suspect this is a simple matter of GNOME Shell can't run under XDMCP because it requires OpenGL which is not longer supported for remote X11 logins. These errors in your log suggest exactly that:
Jan 1 10:27:01 fractal org.gnome.Shell.desktop[8824]: libGL error: No matching fbConfigs or visuals found To confirm it, please report the bug to the developers at https://gitlab.gnome.org/GNOME/gdm/issues ** Changed in: gdm3 (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1956159 Title: XDMCP sessions show "Oh no!" screen instead of greeter To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1956159/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
